An electric arc-blast circuit breaker has at least two arc contacts axially mobile in relation to each other, between a circuit breaker opening position in which the arc contacts are separated from each other and a circuit breaker closing position in which the arc contacts are in contact with each other, an electric arc-blast nozzle and an electric arc cut-off gas circulating in the nozzle to cut an electric arc that is likely to be formed during the movement of the arc contacts from the closing position to the opening position of the circuit breaker.
A conventional electric arc-blast nozzle consists of the following parts:    i. a median neck-forming part internally defining an axial electric arc cut-off passage and formed by a dielectric material obtained from a composition consisting of a fluorocarbon polymer matrix, and    ii. two end parts extending on either side of the median part which are respectively intended to receive the arc contacts that can be axially moved in relation to each other, between a circuit breaker opening position in which the arc contacts are separated from each other and a circuit breaker closing position in which the arc contacts are in contact with each other and in which one of the arc contacts partially closes the axial passage of the median part, an electric arc cut-off gas circulating in the axial passage of the median part to cut an electric arc that is likely to be formed during the movement of the arc contacts from the closing position to the opening position of the circuit breaker.
The dielectric material of the median part of the nozzle is classically obtained from a composition consisting of a fluorocarbon polymer matrix, such as polytetrafluoroethylene (PTFE).
To cut an electric arc, an arc-blast circuit breaker uses a cut-off gas formed by an insulating dielectric gas. This cut-off gas is delivered from a blast chamber in the axial passage of the median part of an electric arc-blast nozzle as described above. The function of such a nozzle is to channel the electric arc and, in doing so, increase the pressure of the cut-off gas around the electric arc, thus encouraging its cut-off.
Currently, the cut-off gas most commonly used in this type of circuit breakers is sulfur hexafluoride SF6 and this, because of its exceptional physical properties. However, SF6 has the major disadvantage of being a very powerful greenhouse gas, with a particularly high global warning potential (GWP).
Among the alternatives to using SF6 as cut-off gas, there are various known gases with lower global warning potential (GWP) than that of SF6, such as dry air or even nitrogen.
Carbon dioxide CO2 is a particularly interesting cut-off gas due to its strong electric insulation and electric arc extinguishing ability. Furthermore, CO2 is nontoxic, non-inflammable, has a very low GWP and, in addition, is easy to procure.
CO2 can be used by itself or in the form of a gaseous mix, constituted mainly of the predominant gas known as “vector gas”.
Since the density of CO2 is lower than that of SF6 and the speed of sound in CO2 is greater than that in SF6, it is observed that the blasting pressure of the electric arc decreases earlier and more quickly with CO2 than with SF6 as the cut-off gas.
Due to this relatively quicker decline in the blasting pressure of the electric arc with CO2, short-circuiting with CO2 is more difficult to achieve than with SF6, specially on long electric arcing times. Under these conditions, the blasting pressure of CO2 may not be sufficient to enable the electric arc cut-off.
In order to overcome this drawback and allow effective electric arc cut-off, the blasting pressure of the electric arc must necessarily be higher when using CO2, instead of SF6, as the cut-off gas.
Multiple solutions were proposed to increase this electric arc-blasting pressure, and thus avoid loss of pressure on long arcing times.
A first solution consists of offering a circuit breaker working with CO2 equipped with a larger swabbing volume than a circuit breaker working with SF6. Thus, such a circuit breaker working with CO2 has an enlarged section of the piston, which requires an increase in the control energy in order to obtain adequate blasting pressure for cutting the electric arc.
The drawback of this first solution resides in the fact that such a circuit breaker has, by construction, larger dimensions than a conventional circuit breaker working with SF6, thus making the circuit breaker working with CO2 more expensive than the one working with SF6.
A second solution consists of using electric arc energy to increase thermal effect, and thus the pressure in the blasting chamber, such as to reinforce the blasting of the arc over long arcing times. This increased thermal effect is possible by confining the electric arc cut-off zone. To this effect, the section of the axial passage for cutting the electric arc of the median part of the nozzle is reduced to encourage the increase in pressure of the cut-off gas in the blasting chamber and increase the blasting pressure of this cut-off gas in this axial passage for cutting the arc.
The drawback of this second solution resides in the fact that strong erosion of the material constituting the nozzle, classically made up of PTFE, is observed for high arc energies during the short-circuiting. If the choice of the PTFE contributes to the increase in pressure of the blasting chamber by degassing and injection of ablated vapors, made up mainly of C2F4 and MoS2, with the action of intense radiation of the electric arc, nevertheless, the section of the axial passage for cutting the median part of the nozzle increases sharply with wear and tear, therefore allocating the cut-off capacity of the circuit-breaker after multiple cut-offs.